Extraoral Taste Buds on the Paired Fins of Damselfishes.

Some fish species have taste buds on the surface of their bodies and fins, as well as in the oral cavity. The extraoral taste system of fish has traditionally been studied in species that inhabit environments and/or employ feeding strategies where vision is limited. Here we examined taste sensation in a new ecological context by investigating the paired fins of damselfish (Pomacentridae), a group of diurnal midwater fishes that inhabit the light-rich waters of coral reefs. Immunohistochemistry demonstrated the presence of taste buds on the paired fins of Chromis viridis, including on the distal tips of elongate leading-edge pelvic fin rays, where they are particularly densely packed, suggesting specialization for chemosensation. Similar anatomical results were also recorded from two other species, Pomacentrus amboinensis and Pomacentrus coelestis. We found that afferent pectoral fin nerves of C. viridis responded to a food-derived stimulus. By investigating the extraoral taste system in a new phylogenetic and ecological context, these results show that taste buds on fins are more widespread amongst fish than previously known and are present even in highly visual environments.

Sensing the structural characteristics of surfaces: texture encoding by a bottom-dwelling fish.

The texture of contacted surfaces influences our perception of the physical environment and modulates behavior. Texture perception and its neural encoding mechanisms have traditionally been studied in the primate hand, yet animals of all types live in richly textured environments and regularly interact with textured surfaces. Here we explore texture sensation in a different type of vertebrate limb by investigating touch and potential texture encoding mechanisms in the pectoral fins of fishes, the forelimb homologs. We investigated the pectoral fins of the round goby (Neogobius melanostomus), a bottom-dwelling species that lives on substrate types of varying roughness and whose fins frequently contact the bottom. Analysis shows that the receptive field sizes of fin ray afferents are small and afferents exhibit response properties to tactile motion that are consistent with those of primates and other animals studied previously. In response to a periodic stimulus (coarse gratings), afferents phase lock to the stimulus temporal frequency and thus can provide information about surface texture. These data demonstrate that fish can have the capability to sense the tactile features of their near range physical environment with fins.

Fins as Mechanosensors for Movement and Touch-Related Behaviors.

Mechanosensation is a universal feature of animals that is essential for behavior, allowing detection of animals' own body movement and position as well as physical characteristics of the environment. The extraordinary morphological and behavioral diversity that exists across fish species provide rich opportunities for comparative mechanosensory studies in fins. The fins of fishes have been found to function as proprioceptors, by providing feedback on fin ray position and movement, and as tactile sensors, by encoding pressures applied to the fin surface. Across fish species, and among fins, the afferent response is remarkably consistent, suggesting that the ability of fin rays and membrane to sense deformation is a fundamental feature of fish fins. While fin mechanosensation has been known in select, often highly specialized, species for decades, only in the last decade have we explored mechanosensation in typical propulsive fins and considered its role in behavior, particularly locomotion. In this paper, we synthesize the current understanding of the anatomy and physiology of fin mechanosensation, looking toward key directions for research. We argue that a mechanosensory perspective informs studies of fin-based propulsion and other fin-driven behaviors and should be considered in the interpretation of fin morphology and behavior. In addition, we compare the mechanosensory system innervating the fins of fishes to the systems innervating the limbs of mammals and wings of insects in order to identify shared mechanosensory strategies and how different organisms have evolved to meet similar functional challenges. Finally, we discuss how understanding the biological organization and function of fin sensors can inform the design of control systems for engineered fins and fin-driven robotics.

A comparison of pectoral fin ray morphology and its impact on fin ray flexural stiffness in labriform swimmers.

The organization of tissues in appendages often affects their mechanical properties and function. In the fish family Labridae, swimming behavior is associated with pectoral fin flexural stiffness and morphology, where fins range on a continuum from stiff to relatively flexible fins. Across this diversity, pectoral fin flexural stiffness decreases exponentially along the length of any given fin ray, and ray stiffness decreases along the chord of the fin from the leading to trailing edge. In this study, we examine the morphological properties of fin rays, including the effective modulus in bending (E), second moment of area (I), segmentation, and branching patterns, and their impact on fin ray stiffness. We quantify intrinsic pectoral fin ray stiffness in similarly sized fins of two closely related species that employ fins of divergent mechanics, the flapping Gomphosus varius and the rowing Halichoeres bivittatus. While segmentation patterns and E were similar between species, measurements of I and the number of fin ray branch nodes were greater in G. varius than in H. bivittatus. A multiple regression model found that of these variables, I was always significantly correlated with fin ray flexural stiffness and that variation in I always explained the majority of the variation in flexural stiffness. Thus, while most of the morphological variables quantified in this study correlate with fin ray flexural stiffness, second moment of area is the greatest factor contributing to variation in flexural stiffness. Further, interspecific variation in fin ray branching pattern could be used as a means of tuning the effective stiffness of the fin webbing to differences in swimming behavior and hydrodynamics. The comparison of these results to other systems begins to unveil fundamental morphological features of biological beams and yields insight into the role of mechanical properties in fin deformation for aquatic locomotion.

The relationship between pectoral fin ray stiffness and swimming behavior in Labridae: insights into design, performance and ecology.

The functional capabilities of flexible, propulsive appendages are directly influenced by their mechanical properties. The fins of fishes have undergone extraordinary evolutionary diversification in structure and function, which raises questions of how fin mechanics relate to swimming behavior. In the fish family Labridae, pectoral fin swimming behavior ranges from rowing to flapping. Rowers are more maneuverable than flappers, but flappers generate greater thrust at high speeds and achieve greater mechanical efficiency at all speeds. Interspecific differences in hydrodynamic capability are largely dependent on fin kinematics and deformation, and are expected to correlate with fin stiffness. Here we examine fin ray stiffness in two closely related species that employ divergent swimming behaviors, the flapping Gomphosus varius and the rowing Halichoeres bivittatus To determine the spatial distribution of flexural stiffness across the fin, we performed three-point bending tests at the center of the proximal, middle and distal regions of four equally spaced fin rays. Pectoral fin ray flexural stiffness ranged from 0.0001 to 1.5109 µN m2, and the proximal regions of G. varius fin rays were nearly an order of magnitude stiffer than those of H. bivittatus In both species, fin ray flexural stiffness decreased exponentially along the proximodistal span of fin rays, and flexural stiffness decreased along the fin chord from the leading to the trailing edge. Furthermore, the proportion of fin area occupied by fin rays was significantly greater in G. varius than in H. bivittatus, suggesting that the proportion of fin ray to fin area contributes to differences in fin mechanics.

Touch sensation by pectoral fins of the catfish Pimelodus pictus.

Mechanosensation is fundamental to many tetrapod limb functions, yet it remains largely uninvestigated in the paired fins of fishes, limb homologues. Here we examine whether membranous fins may function as passive structures for touch sensation. We investigate the pectoral fins of the pictus catfish (Pimelodus pictus), a species that lives in close association with the benthic substrate and whose fins are positioned near its ventral margin. Kinematic analysis shows that the pectoral fins are held partially protracted during routine forward swimming and do not appear to generate propulsive force. Immunohistochemistry reveals that the fins are highly innervated, and we observe putative mechanoreceptors at nerve fibre endings. To test for the ability to sense mechanical perturbations, activity of fin ray nerve fibres was recorded in response to touch and bend stimulation. Both pressure and light surface brushing generated afferent nerve activity. Fin ray nerves also respond to bending of the rays. These data demonstrate for the first time that membranous fins can function as passive mechanosensors. We suggest that touch-sensitive fins may be widespread in fishes that maintain a close association with the bottom substrate.

Distinct roles for protein kinase C isoforms in regulating platelet purinergic receptor function.

ADP is a critical regulator of platelet activation, mediating its actions through two G protein-coupled receptors (GPCRs), P2Y1 and P2Y12. We have shown previously that the receptors are functionally desensitized, in a homologous manner, by distinct kinase-dependent mechanisms in which P2Y1 is regulated by protein kinase C (PKC) and P2Y12 by G protein-coupled receptor kinases. In this study, we addressed whether different PKC isoforms play different roles in regulating the trafficking and activity of these two GPCRs. Expression of PKCalpha and PKCdelta dominant-negative mutants in 1321N1 cells revealed that both isoforms regulated P2Y1 receptor signaling and trafficking, although only PKCdelta was capable of regulating P2Y12, in experiments in which PKC was directly activated by the phorbol ester phorbol 12-myristate 13-acetate (PMA). These results were paralleled in human platelets, in which PMA reduced subsequent ADP-induced P2Y1 and P2Y12 receptor signaling. PKC isoform-selective inhibitors revealed that novel, but not conventional, isoforms of PKC regulate P2Y12 function, whereas both novel and classic isoforms regulate P2Y1 activity. It is also noteworthy that we studied receptor internalization in platelets by a radioligand binding approach showing that both receptors internalize rapidly in these cells. ADP-induced P2Y1 receptor internalization is attenuated by PKC inhibitors, whereas that of the P2Y12 receptor is unaffected. Both P2Y1 and P2Y12 receptors can also undergo PMA-stimulated internalization, and here again, novel but not classic PKCs regulate P2Y12, whereas both novel and classic isoforms regulate P2Y1 internalization. This study therefore is the first to reveal distinct roles for PKC isoforms in the regulation of platelet P2Y receptor function and trafficking.

Evidence that the purinergic receptor P2Y12 potentiates platelet shape change by a Rho kinase-dependent mechanism.

ADP activates human platelets through two G-protein coupled receptors, P2Y1 and P2Y12, to induce a range of functional responses. Here we have addressed the role and mechanism of P2Y12 in modulating ADP-induced platelet shape change. Although the response depended upon activation of P2Y1, it was potentiated by P2Y12 as the P2Y12-selective antagonists AR-C69931MX and 2MeSAMP partially inhibited shape change in the later phase of the response. This was paralleled by inhibition of pseudopod formation, platelet spheration, actin polymerisation and myosin light chain phosphorylation. P2Y12 is known to couple to activation of PI3 kinase and inhibition of adenylate cyclase, but we showed that neither of these signalling events couples to regulation of shape change by this receptor. However, by assessment of phosphorylation of its major substrate myosin light chain phosphatase, we provide direct evidence for activation of Rho kinase by ADP, and that although P2Y1 is required for activation of Rho kinase, P2Y12 is able to potentiate its activity. We conclude that P2Y12 plays a potentiatory role in ADP-induced shape change through regulation of the Rho kinase pathway, potentiating both myosin phosphorylation and actin polymerisation, and this forms part of an important signalling pathway additional to its well-established Gi-coupled pathways.

P2Y1 and P2Y12 receptors for ADP desensitize by distinct kinase-dependent mechanisms.

Adenosine 5'-diphosphate (ADP) plays a central role in regulating platelet function by the activation of the G protein-coupled receptors P2Y(1) and P2Y(12). Although it is well established that aggregation responses of platelets to ADP desensitize, the underlying mechanisms involved remain unclear. In this study we demonstrate that P2Y(1)- and P2Y(12)-mediated platelet responses desensitize rapidly. Furthermore, we have established that these receptors desensitize by different kinase-dependent mechanisms. G protein-coupled receptor kinase (GRK) 2 and GRK6 are both endogenously expressed in platelets. Transient overexpression of dominant-negative mutants of these kinases or reductions in endogenous GRK expression by the use of specific siRNAs in 1321N1 cells showed that P2Y(12), but not P2Y(1), desensitization is mediated by GRKs. In contrast, desensitization of P2Y(1), but not P2Y(12), is largely dependent on protein kinase C activity. This study is the first to show that both P2Y(1) and P2Y(12) desensitize in human platelets, and it reveals ways in which their sensitivity to ADP may be differentially and independently altered.

Reciprocal cross-talk between P2Y1 and P2Y12 receptors at the level of calcium signaling in human platelets.

Adenosine diphosphate (ADP), an important platelet agonist, acts through 2 G-protein-coupled receptors (GPCRs), P2Y(1) and P2Y(12), which signal through Gq and Gi, respectively. There is increasing evidence for cross-talk between signaling pathways downstream of GPCRs and here we demonstrate cross-talk between these 2 ADP receptors in human platelets. We show that P2Y(12) contributes to platelet signaling by potentiating the P2Y(1)-induced calcium response. This potentiation is mediated by 2 mechanisms: inhibition of adenylate cyclase and activation of phosphatidylinositol 3 (PI 3)-kinase. Furthermore, the Src family kinase inhibitor PP1 selectively potentiates the contribution to the calcium response by P2Y(12), although inhibition of adenylate cyclase by P2Y(12) is unaffected. Using PP1 in combination with the inhibitor of PI 3-kinase LY294002, we show that Src negatively regulates the PI 3-kinase-mediated component of the P2Y(12) calcium response. Finally, we were able to show that Src kinase is activated through P2Y(1) but not P2Y(12). Taken together, we present evidence for a complex signaling interplay between P2Y(1) and P2Y(12), where P2Y(12) is able to positively regulate P2Y(1) action and P2Y(1) negatively regulates this action of P2Y(12). It is likely that this interplay between receptors plays an important role in maintaining the delicate balance between platelet activation and inhibition during normal hemostasis.